A ringback tone (or ringing tone) is an audible indication that is heard on the telephone line by the caller while the phone they are calling is being rung. It is normally a repeated tone, designed to assure the calling party that the called party's line is ringing, although the ring-back tone may be out of sync with the ringing signal.
The ringback tone is in most cases generated in a distant switch and transmitted in-band. The distant switch also sends a message out band indicating to the rest of the following network that the phone is ringing. In most public phone networks today the ringback tone is not generated in the handset or by the local switch, as customized tones or voice announcements may be generated by the distant switch in place of a ringing signal.
Some carriers allow a subscriber to customize the ringback tone heard by some or all callers to the subscriber, such that when a caller calls the subscriber, the caller does not hear the normal repeated tone, but instead hears music, a message from the subscriber, or other recording.